There is known, as an assembly used for a catalytic converter or a filter for purification of exhaust gas of internal combustion engine or chemical reaction equipment utilizing catalytic action, a heat exchanger, or the like, said assembly comprising a cell structure being housed and held (canned) in a metal vessel by placing a compressible material having a cushioning property between the cell structure and the tubular metal vessel (can), with applying a predetermined mounting pressure to the cell structure via the compressible material.
For example, in the case where such an assembly is used as a catalytic converter for purifying automobile exhaust gas, a precious metal such as platinum, palladium, or rhodium is dispersedly carried on a ceramic honeycomb structure, which is one kind of cell structure, as a catalyst component, and the honeycomb structure carrying the precious metal is housed and held in a metal vessel (can) via a ceramic mat etc. and is mounted on an exhaust system.
It is desirable that the cell structure has a high strength because it is held in the metal vessel with applying a mounting pressure onto the outer peripheral surface thereof as described above. For the honeycomb structure used as a catalyst carrier for purifying automobile exhaust gas, however, the thinning of cell wall is now in progress for the improvement in the purification performance, and accordingly the strength level is being lowered.
The strength of cell structure can be measured by an “isostatic fracture strength test”. This test is conducted by putting a cell structure in a tubular rubber vessel, placing a cover of aluminum plate on the vessel; and performing isostatic compressing in water; which simulates the compressive load in the case where the outer peripheral surface of cell structure is held by the can of converter. The isostatic strength is shown by a pressure value applied thereto at the time when the cell structure is fractured, and is specified in Automobile Standards JASO Standards M505-87 issued by Society of Automotive Engineers of Japan, Inc.
It has been found that it is generally very difficult for a ceramic honeycomb structure used as a catalyst carrier for an automobile exhaust gas purifying converter to keep an isostatic strength of 10 kg/cm2 or higher if the cell wall thickness is 0.11 mm or smaller and the open frontal area ratio in the terms of percentage exceeds 85%.
In the case where a specific pressure higher than a design specific pressure set at the time of canning design is produced in actual canning and the specific pressure exceeds the isostatic strength of cell structure in some portion, there is a danger that the structure fractures in that portion. As the thickness of cell wall of cell structure becomes thinner and thus the strength level of structure is lowered, the design specific pressure must be lowered, and thus it is necessary to restrain an abnormal rise in specific pressure in actual canning and to reduce the variations in specific pressure as much as possible. If the actual specific pressure is equal to the design specific pressure, intended canning design can be made ideally.
Further, the cell structure may be fractured in the case where because a gap between the cell structure and the metal vessel for containing the cell structure is irregular due to poor accuracy of external shape of cell structure, the compressive pressure acting on the outer peripheral portion of cell structure is nonuniform, so that a high holding specific pressure acts partially.
On the other hand, if the specific pressure is decreased too much, the cell structure cannot be kept being held in the metal vessel because the cell structure is subjected to a high-temperature exhaust gas flow or vibrations in actual service environments. Therefore, the necessary minimum specific pressure exists. As the wall thickness of cell structure decreases, the isostatic strength level of cell structure decreases, so that the mounting pressure for holding the cell structure must also be decreased to the utmost while the minimum specific pressure necessary for holding the cell structure is maintained. As the level of mounting pressure decreases, variations in specific pressure must be decreased, that is, more even specific pressure distribution must be provided.
In order to house and hold the cell structure in the metal vessel in a proper state, it is desirable to decrease the variations in size and shape of the cell structure to the utmost. However, since the ceramic honeycomb structure used as a catalyst carrier as described above is dried as it is after being extrusion molded, and after being cut to a predetermined length, it is housed in the metal vessel in a state of being fired, the outside-diameter dimension of honeycomb structure involves dimensional variations and deformations in all processes of molding, drying, and firing. Therefore, the honeycomb structure has very large variations in size and shape as compared with a metal work.
In some cases, in a large-size honeycomb structure, the outer peripheral portion thereof is machined and removed after firing, and then the outer periphery is covered with ceramic cement. In general, however, the ceramic honeycomb structure is put to practical use without machining the outer peripheral portion thereof after firing. Therefore, studies have been carried out to increase the accuracy of outside diameter of ceramic honeycomb structure, and on the other hand, the problem is how the influence of outside-diameter dimension of ceramic honeycomb structure is kept little when the honeycomb structure is housed in the metal vessel.
The present invention has been made to solve the above problems, and accordingly an object thereof is to provide an assembling method in which when an assembly produced by housing and holding a cell structure in a metal vessel via a compressible material, even if the external-shape dimension of member such as the cell structure constituting the assembly has variations, the influence of variations is averted, and a proper holding state without a fracture etc. of cell structure can be obtained.